asterisk users - Mar 2007 - Ringing does not terminate on mISDN after pickup

Hello,

I am having something of an odd problem: about every 100 calls or so, 
when a call comes in via an external mISDN interface and I route it to 
an internal mISDN interface by dialing an internal msn that is 
programmed for multiple phones on the internal bus, somtimes the other 
phones continue ringing for several minutes after the call has already 
been picked up by one (or even eventually hungup already...). As you 
imagine this is really annoying...

I don't seem to be able to narrow it down in any way: show channels is 
empty (provided, call was answered an hungup, other phones continue ringing)

The only way to terminate the ringing is: wait for several minutes... :-)
or: restart asterisk.

Can anybody point me in the right direction with this issue? Every 
attempt to get rid of the problem has failed. I have no clue what else I 
could try... - Does anybody else experience this as well?

Cheers,
Arik